By definition DC to DC charging is simply using the Direct Current (DC) from one source, be that a battery or alternator, to charge another. There are MANY different styles of DC to DC systems on the market and you can suffer from information overload pretty quickly. In this article we will take a look at the basics and some tips to get us started.

Why do I need a DC to DC charger?

Traditional solenoid or voltage control relays only function to connect your vehicle's charging system to the second battery. Unfortunately THIS WILL NOT FULLY CHARGE BATTERIES. A modern multi-stage DC to DC charger will.

Modern vehicles have complex charging systems that DC to DC chargers are better suited to.

Modern DC to DC chargers are better at charging batteries of dissimilar chemistry. For example you may have a Flooded start battery and a Deep Cycle or AGM second battery. The vehicle's charging system is designed for Flooded batteries so the DC to DC charger adjusts the voltages to better suit Deep Cycle batteries.

Sizing your DC to DC chargerWe can be forgiven for thinking that the bigger the charger the better! After all, the vehicle has a 150 Amp alternator in it, right. Well yes, but that is the absolute maximum output of an alternator and not something that it can sustain for more than a few minutes. The ACTUAL output of an alternator can be considerably lower. We also need to consider that the vehicle itself will consume a lot of power from the system at various times. I can assure you that vehicle manufacturers do not put over sized alternators on cars, rather they put the leanest system possible in place to reduce cost.

So now you come along and stick a 50 Amp DC to DC charger under the bonnet which asks the vehicle's charging system for all 50 Amps. Sadly, more often than not, the charging system simply will not have that available to give. If it can, this excessive load can cause damage to the alternator itself. In fact some manufacturers are seeing high failures in alternators (Delaminating) and are not honouring warranty when dual battery systems are fitted. So choosing the size of your DC to DC charger is something worth considering and remember, bigger is not always better.

If you have any doubts then a quick chat with your local Auto Electrician is a great idea. He may even be able to run some tests on your system to help you determine what is best for you. Don't begrudge the hourly rate especially considering what it can cost you if the system fails.

Choosing the right DC to DC charger Again there are many different types and styles of charger on the market to choose from. So here are some points to consider when purchasing your DC to DC system.

Do I need a solar input and if so, is my solar regulated at the panel or unregulated. Note that some DC to DC chargers come with a built in regulator so don't connect a regulated panel to these.

Where will the unit be mounted and does it need to be water proof or splash proof. Check the IP rating carefully!

What Amperage charger will best suit my vehicle and the batteries I am charging.

Do I need to monitor the temperature of the second battery to prevent over charging.

Is my system 12 or 24 Volts (Passenger cars are 12V but RV and motor homes can be 24V).

Does the unit I am buying have a fan and will this be noisy if mounted in a caravan or camper.

Is the unit I am buying reverse polarity protected and can this be reset if I get it wrong.

Does the unit provide multiple stages of charge to fully charge my second battery.

How big is the unit and where can I mount this (is there enough room).

What else will I need?When it comes to installing your DC to DC you will also need some or all of the following;

Extra Cable (16mm2 is ideal for under bonnet installs)

Lugs to put on the ends of the cables (You will also need a soldering Iron, solder and/or a crimping tool)

Heat shrink to suit the cable and Lug size (A heat gun for the heat shrink)

2 Battery terminals (dependent on the style of battery you use)

Screws to mount the unit

Battery Monitor - Its always a good idea to monitor the voltages of your system (I like to monitor Voltage & Amperage)

If you would like more information on charging and DC to DC systems then have a look at our latest publication "Recreational Power Guide - Batteries and Charging". You can find this in our store www.warbonnet.com.au/store

In all Modern common rail diesels the vehicle manufacturers are utilising some very advanced charging technologies to help reduce engine loads and therefore emissions. The important thing to understand is that the vehicles management system monitors the battery voltage and the load coming off the start battery. The management system then changes the alternator’s charge output to suit the situation. For example if you are cruising along a highway with very little electrical load and the battery is charged, the management system will reduce the alternator’s output to quite low levels so as to reduce load on the engine and save fuel and emissions. I have seen some models get as low as 12.6V. So a problem occurs if we put a typical battery isolator or VCR relay in the vehicle to charge our second battery. You see the management system on the vehicle has no way of measuring the second battery. As far as it is concerned the second battery doesn’t exist. There will be many occasions in the drive that the system will reduce the voltage to well below the pre-set voltages in a VCR (remembering that they typically disconnect at 12.8V). So you are cruising down the road with the fridge running off the second battery but because there is no electrical load coming off the start battery, the alternator’s charge output is reduced, the reduced voltage drops out the VCR relay and your second battery stops charging. There are many tricks being employed to solve this problem such as resistors being placed on the start battery to trick the vehicle into thinking it has a load.DC TO DC CHARGERS A DC to DC charger uses voltage from one battery to charge another and typically they do not connect to 240V (although some models are coming on the market with this additional feature). DC to DC chargers solve some of the following charging dilemmas:Charging Different Battery TypesAs we learned earlier there are many different battery styles (Flooded, AGM, GEL etc) and each type has its own quirks when it comes to charging. Flooded batteries which are the most common vehicle batteries typically charge at around 13.8V-14.1V in a vehicle however if we put an AGM in as the second battery they like to charge at a much higher voltage so straight away we have a problem. A quality DC to DC charger will step up the voltage being sent to the second battery so that it receives the appropriate charge voltage and it will apply a Bulk, Absorption and Float charge. Something a VCR can never do!Alternators Do Not Fully Charge Vehicle BatteriesMost of you will be shocked to know that your fully charged car battery is only about 80% charged. A battery needs to go through several stages of charge to achieve full saturation. A vehicle’s charging system is just not capable of providing the right levels of charge. Therefore an Isolator or VCR will also fall short. Most modern DC to DC chargers will provide these additional charging stages so that you get the full potential from your second battery. After all, what would be the point of buying an expensive second battery if you are only going to get access to 80 – 85% of its capacity? Long Cable Runs One big advantage DC to DC chargers have over a traditional VCR is that they step up the voltage to better suit Deep Cycle batteries. This step up in voltage can also assist to recover voltage drop in longer cable runs. This becomes a problem when the second battery is in the boot or mounted in a caravan or camper trailer.

This has been an extract from our latest publication "Recreational Power Guide - Batteries & Charging". For more information on charging multiple batteries, Dual Battery systems and charging visit our web site at www.warbonnet.com.au or check out our latest publication Recreational Power Guide - Batteries & Charging at www.warbonnet.com.au/store